1. Field of the Invention
The present invention relates to an optical head positioning apparatus.
2. Description of the Related Art
An electrophotographic printer that employs an LED head incorporates a photoconductive drum, which is positioned so that a charged surface of the photoconductive drum is at a focal point of a convergent lens such as a SELFOC lens array (SLA). During an exposure process of the electrophotographic printer, light emitted from LED array chips illuminates the surface of the photoconductive drum through the SLA to form an electrostatic latent image on the surface.
FIG. 35 is a cross-sectional view of a conventional LED head.
An LED array chip 1 is mounted on a printed circuit board 3. An SLA holder 4 holds an SLA 2 thereon. A base 5 holds the printed circuit board 3, SLA holder 4, and SLA 2 thereon and accurately positions them relative to the surface of the photoconductive drum 6. In order to focus an image on the photoconductive drum 6, the LED head requires to be accurately positioned with respect to the photoconductive drum 6. Thus, the LED head is positioned so that a distance Lo from the LED chip 1 to a light-entering surface of the SLA 2 is equal to a distance Li from the light-exiting surface of the SLA 2 to a focal point on the photoconductive drum 6. The SLA 2 is the distance Lo away from the LED array chip 1 and is fixed to the SLA holder 4 by an adhesive. In other words, the distance Lo cannot be adjusted once the SLA 2 has been mounted on the SLA holder 4. Thus, the photoconductive drum 6 should be positioned accurately relative to the LED head so that the distance Lo is equal to the distance Li.
FIG. 36 is a front view of the conventional LED head.
The positional relation between the conventional photoconductive drum 6 and the LED head will be described with reference to FIG. 36. The photoconductive drum 6 has one axial end to which a gear 7 is mounted and the other axial end to which a flange 11 is mounted. The gear 7 and flange 11 are formed with a hole 9 and a hole 10 therein, respectively, through which a shaft 8 of the photoconductive drum 6 extends. The gear 7 and flange 11 rotate on the shaft 8. The gear 7 is driven in rotation by a drive source, not shown, thereby driving the photoconductive drum 6 to rotate.
The photoconductive drum 6 is disposed in an ID unit, not shown, and is covered with an upper frame 16 such that the photoconductive drum 6 is shielded from light except a surface area that opposes the light-exiting end of the SLA 2. The shaft 8 is rotatably supported at its longitudinal end portions by side frames 12a and 12b of the ID unit. Adjusting mechanisms 13a and 13b are disposed under both end of the SLA holder 4 and operated to adjust the distance Li between the light-exiting end of the SLA 2 and the surface of the photoconductive drum 6.
The adjusting mechanisms are fixed permanently after adjusting the distances Lo and Li. The LED head is urged toward the shaft 8 of the photoconductive drum 6 by springs 14a and 14b, which are mounted on an upper portion of the both end portions of the LED head. The adjusting mechanisms 13a and 13b abut abutting surfaces 15a and 15b formed on the side frames 12a and 12b. The adjusting mechanisms 13a and 13b maintain the distance Li at a fixed value so that light is focused on the surface of the photoconductive drum 6.
The conventional apparatus of the aforementioned construction suffers from the following drawbacks. The distance Li is adjusted with the LED head mounted on a jig. When the thus adjusted LED head is assembled to a printer, the adjusting mechanisms 13a and 13b abut the abutting surfaces 15a and 15b of the side frames 12a and 12b in the ID unit. At this moment, the distance Li changes slightly so that a focal position deviates somewhat from its correct position, preventing formation of well focused images.
This is due to the fact that the distance of the photoconductive drum 6 from the SLA 2 deviates from a designed value Li. The deviation of the distance is within xc2x1100 xcexcm of the designed Li. The factors that cause the manufacturing variations of Li primarily include tolerances of the shaft 8, holes 9 and 10, the height of the abutting surfaces 15a and 15b, and the wear of the photoconductive drum 6. For this reason, the adjusting mechanisms 13a and 13b of each ID unit are adjusted to a corresponding ID unit when the IED head is assembled to the ID unit. However, ID unit is a consumable item. When the ID unit reaches the end of its useful life, the user replaces the ID unit by a new, unused one. Thus, after the ID unit is replaced, the distance Li between the SLA 2 and the surface of the photoconductive drum 6 may be different from that before the ID unit is replaced.
FIG. 37 illustrates the relationship between xcex94Li and MTF (Modulation Transfer Function). The closer to 100% the MTF is, the more faithful to an original image the printed image is. From FIG. 37, it can be seen that a deviation of Li of 50 xcexcm causes a decrease of MTF of more than 10%. For a printer having a resolution of 1200 DPI (about 24 line pairs/mm), a decrease of MTF in excess of 10% impairs the resolution of a printed image.
An object of the invention is to solve the aforementioned problem.
Another object of the invention is to improve the accuracy of positioning of an LED head with respect to the surface of a photoconductive drum so that an image is focused accurately on the surface of the photoconductive drum.
A positioning apparatus for an optical head includes a cylindrical photoconductive drum, an optical head, and at least one spacer. The cylindrical photoconductive drum extends in a direction of a longitudinal axis. The optical head extends parallel to the photoconductive drum. The at least one spacer is disposed to abut the photoconductive drum, the spacer limiting a distance between the optical head and a surface of the photoconductive drum.
The photoconductive drum has a photoconductor and the spacer is contact with the surface of the photoconductor through sliding friction.
The spacer has a first surface in contact with the surface of the photoconductor. The first surface has a groove formed therein.
The photoconductor has a second surface in contact with the first surface. The first surface has a first curvature and the second surface has a second curvature. When the first surface is pressed against the second surface, the spacer deforms resiliently so that the first curvature becomes substantially equal to the second curvature.
The electrophotographic printer further includes a charging roller that extends in a direction in which the photoconductive drum extends, the charging roller being in contact with the photoconductor. The spacer is located outside of an area in which the charging roller is in contact with the photoconductor.
The photoconductive drum has a member coaxial with the photoconductive drum and rotates in contact with the first surface together with the photoconductive drum.
The second surface of the spacer is on an opposite side from the first surface. The electrophotographic printer further includes an adjusting mechanism that is held sandwiched between the optical head, and the second surface having the second curvature. The adjusting mechanism is operated to adjust the position of the optical head relative to the photoconductive drum. The adjusting mechanism may be an eccentric cam mechanism.
The first surface and the second surface have a curvature and are concentric to each other.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.